1. Field of the Invention
The present invention relates to an image forming apparatus having a transfer unit for transferring to a recording medium a toner image developed on a photoreceptor.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic process generally includes a photoreceptor serving as an image bearing member which rotates, and a charging section, an exposure section, a developing section, a transfer section, a fixing section, a cleaning section, and an electricity removing section which are arranged around the photoreceptor. The charging section uniformly charges a surface of the photoreceptor. The exposure section irradiates the charged surface of the photoreceptor with light according to the image information and thereby forms an electrostatic latent image. The developing section stirs a toner therein to cause friction which electrifies the toner so that the toner is attached to the electrostatic latent image formed on the surface of the photoreceptor, and thereby forms a toner image. The transfer section provides a recording medium with charges of which polarity is opposite to that of the charges in the toner and thereby transfers the toner image to the recording medium. The fixing section fixes the transferred toner image to the recording medium with heat, pressure, or the like means. The cleaning section collects the toner which has failed to be transferred and thus remains on the surface of the photoreceptor. The electricity removing section removes the electricity of the photoreceptor, which remains after transferring of the toner image. The image forming apparatus employing the electrophotographic process constructed as above forms desired images on recording mediums.
A transfer unit, which serves as the transfer section for transferring the toner image from the photoreceptor to the recording medium, is composed of a transfer belt, a plurality of rollers on which the transfer belt is suspended in a tensioned manner and which turns the transfer belt around, and a transfer roller to which is applied an electric field having an opposite polarity to that of the charges of the toner image on the surface of the photoreceptor so that the toner image is transferred to recording paper on the transfer belt. The toner image on the photoreceptor is transferred to the recording paper conveyed to the running transfer belt.
In forming a color image in the electrophotographic process, toners of plural colors; for example, black (K), cyan (C), magenta (M), and yellow (Y), are used. In a color image forming apparatus, a structure having a photoreceptor as a center with other various sections such as those stated above, is provided for each color and a toner image is formed for every color. The toner images of all colors are transferred to a recording medium to be overlaid with each other and then fixed to the recording medium. A color image can be thus formed by subtractive color mixture.
To an intermediate transfer belt, toner images of respective colors are transferred in a manner that these images are sequentially overlaid with each other, thereby forming a multicolored toner image which will be then transferred to a recording medium.
In order to form images in such constitution as above, it is necessary to accurately adjust a rotating speed of each photoreceptor and a running speed of the transfer belt. An adjustment failure will lead to such transfer misregistration as position displacement of an image transferred to a recording medium, or deviation of image magnification, and in the case of forming color images, misregistration in overlaying toner images.
In an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 11-109827 (1999), one of a scanning speed of an exposure section, a rotating speed of an image bearing member, and a revolving speed of an intermediate transfer member, is controlled based on a count value, i.e., the number of sheets on which images have been printed, a temperature, and the like factor, in order to reduce fluctuation of the image magnification depending on durability of the intermediate transfer member as well as to reduce image change due to variation in electric resistance of the intermediate transfer member.
In an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 2004-252295, a revolving speed of a belt is directly measured at a transfer position and on the basis of the measurement result, the moving speed of the belt is controlled, in order to reduce position displacement of an image in spite of a change in thickness of, the elongation and vibration, etc. of the belt.
A toner may be attached to not only a photoreceptor but also a transfer belt, and this is why a cleaning section for transfer belt is provided to remove the toner attached to the transfer belt. A commonly-known cleaning section is composed of a cleaning blade which is made of an elastic material such as urethane rubber and disposed in contact with a surface of the transfer belt so as to scrape off the toner attached thereto.
For the transfer belt, a material having surface roughness Rz of around 6 μm is selected, and as the cleaning operation is performed with the cleaning blade in contact with the transfer belt, its microabrasion action undesirably causes the transfer belt to be gradually abraded away, resulting in the transfer belt having the surface roughness Rz of around 1 μm.
As the transfer belt has decreased surface roughness Rz, a frictional force increases at a contact portion between the cleaning blade and the transfer belt, and the cleaning blade may work as a brake, lowering the running speed of the transfer belt.
This causes mismatching between the rotating speed of the photoreceptor and the running speed of the transfer belt, which leads to some transfer misregistration.
In the image forming apparatus disclosed in JP-A 11-109827, the speed deterioration of the transfer belt due to the change in its surface roughness is not taken into consideration. In the image forming apparatus disclosed in JP-A 2004-252295 in which the speed of the belt is directly measured, it is possible to deal with the speed deterioration of the transfer belt due to the change in its surface roughness, but at the same time, it is necessary to provide a measurement section for directly measuring the speed of the belt.